The invention relates to a system and a method for controlling a positioning device, in particular actuators pertaining to machine tools, with an incremental position measuring device with output signals of the form sin xcfx861, sin (xcfx861+p) and a control unit converting the output signals into control signals and consequently controlling the positioning device. In this case, p is an angle by which the two output signals are phase-shifted.
The invention also relates to the use of a device, which can be used with said system or when carrying out said method, as a phase mixing device.
In the case of machine tools, processing machines and production machines, but also in the case of various measuring and testing devices, it is necessary to position a tool or a measuring sensor automatically in relation to a workpiece or a part to be tested. For reasons of simplicity, the invention is always described below with reference to the example of positioning a tool of a machine tool in relation to a workpiece, without however restricting the invention to this specific application.
For the positioning of tool and workpiece, the tools on modern machine tools can generally be moved and swiveled or rotated about five or more axes. The moving and swiveling or rotating movements are controlled by a central control unit, generally what is known as numerical control (NC), the control unit continually receiving information from a position measuring device about the positional changes made during the moving, swiveling or rotating.
Among the position measuring devices, what are known as incremental (counting) position measuring devices have proven successful in particular. Depending on the type of output signals, a distinction is drawn here between measuring devices with rectangular (and consequently quasi digitized) output signals (the signals generally being transmitted in the form of a voltage pulse) and those with sinusoidal (that is analog) output signals. The devices with rectangular output signals are distinguished by relatively simple signal processing, the positional deviations generally amounting to xc2x13% of the signal period.
If traversing rates of at least 0.01 m/min are to be realized with a relatively short sampling time of, for example, 250 xcexcs, and if, for reasons of accuracy, a change of at least one measuring step per sampling cycle is to take place, a measuring step of only 0.04 xcexcm is required. With this measuring step, at 60 m/min for example, a step frequency of 60 m/(60s*0.04 xcexcm)=25 MHz is obtained. To keep down the complexity of the circuitry in the downstream electronics, input frequencies of less than 1 MHz are desirable. For such high traversing rates and very small measuring steps, incremental position measuring devices with sinusoidal output signals have proven to be highly successful, a, relative positional deviation within a signal period for drives with digital speed control of less than 1% of the respective signal period of the measuring device making said devices even more accurate than measuring devices with rectangular output signals.
However, there is the problem that, on account of. Nonlinearities, thermal expansions, vibrations etc., the workpiece and tool are in fact in a position in relation to each other which deviates from the position which the control unit has calculated on the basis of the signals transmitted to it by the measuring device. Although such deviations are often only very small, in cases in which extremely small production tolerances are prescribed for the machining of a workpiece they can lead to the complete workpiece becoming unusable. In this case, the deviations occurring under certain conditions and/or in certain positions can often be calculated or measured in advance.
DE 32 01 005 C2 discloses a system for controlling a positioning device, in particular actuators pertaining to machine tools, which comprises an incremental position measuring device with output signals of the form sin xcfx861, sin (xcfx861xc2x1p), p being an angle by which the two output signals are phase-shifted, a control unit converting the output signals into control signals and consequently controlling a positioning device, and means for mixing a correction angle xcfx862 into the output signals, the means being connected between the position measuring device and the control unit in such a way that the output signals obtain the form sin (xcfx861xc2x1xcfx862), sin (xcfx861xc2x1xcfx862xc2x1p) before they are converted n control signals by the control unit.
However, this system does not allow arbitrary correction angles to be taken into account and consequently cannot be used for controlling the positioning device.
DE 32 01 005 C2 stipulates that it is absolutely necessary to provide correction tracks alongside a scale, which have to be scanned for the acquisition of correction signals. According to the teaching of this document, it is consequently always absolutely necessary, for at least two xe2x80x9cscalesxe2x80x9d to be scanned, that is to say firstly the actual scale and the associated correction track. The correction angles are consequently not arbitrary, but result exclusively from the scanned values of the correction track.
GB 2 335 987 A1 discloses a method for calibrating and compensating errors which are in the measuring system itself. The aim here is to eliminate xe2x80x9cshort range errorsxe2x80x9d, in also being possible for these errors to be xe2x80x9cphase errorsxe2x80x9d, which have to be initially determined by means of FFT analysis and then used for correction. However, a machine in the sense of the present application cannot be controlled by calibrating or compensating the errors.
WO 98/00921 discloses an angle encoder for throttle valves. In the evaluation unit belonging to the encoder, the angle of the encoder is determined by mixing and multiplying harmonics of the excitation signal fed in. However, this evaluation unit is not a phase mixer in the sense of the present application.
The document U.S. Pat. No. 5,414,516 discloses a method for the high-resolving evaluation of the angle of two sinusoidal measuring signals, which are respectively offset by 90xc2x0. This document does not teach the active control of a positioning device by mixing in a correction angle.
The document EP 0 652 419 A1 discloses a method for the compensation of phase errors in measuring systems, it being possible to take into account angle errors determined in advance or else online, independently of the angle of the measuring system determined at the particular time, in the further processing by angle addition. However, the method does not allow arbitrary angles to be mixed into the measuring system. Rather, only errors within a graduation period are corrected here.
The document U.S. Pat. No. 5,375,066 discloses an apparatus for mixing in correction angles, which can be used only in the case of digital measuring systems or measuring systems with TTL level and not in the case of analog, sinusoidal measuring system signals which are phase-shifted.
Against this background, the invention is based on the object of specifying a device and a method for controlling a positioning device in which certain positional deviations can be automatically taken into account, without changes having to be made to the control units which, in themselves, have proven successful, or their mode of operation. In particular, it is intended that arbitrary correction angles can be taken into account, so that they can theoretically also be used for controlling the positioning device.
The object is achieved by a system for controlling a positioning device, in particular actuators pertaining to machine tools, with an incremental position measuring device with output signals of the form sine, sin (xcfx861xc2x1p), p being an angle by which the two output signals are phase-shifted, a control unit converting the output signals into control signals and consequently controlling a positioning device, and means for mixing a correction angle xcfx862 into the output signals, the means being connected between the position measuring device and the control unit in such a way that the output signals obtain the form sin (xcfx861xc2x1xcfx862), sin (xcfx861xc2x1xcfx862xc2x1p) before they are converted into control signals by the control unit, the system being characterized in that the means for mixing in a correction angle can access at least one freely programmable correction angle table with values of the correction angles xcfx862 to be taken into account for specific states x, or their trigonometric equivalents.
The invention is consequently based on the basic idea of xe2x80x9cinterceptingxe2x80x9d the position signals supplied by the respective position measuring device on their way to the control unit, changing them in accordance with prescribed criteria and passing the changed position signals to the control unit, in order in this way to correct deviations between the measured position and the actual position of the tool or the like to be controlled. The correction angle xcfx862 to be mixed in here mayxe2x80x94depending on the type of position device to be controlled and depending on the respective special conditionsxe2x80x94even be far greater than 2xcfx80 or than 360xc2x0, so that not only deviations within a step interval but also deviations of any magnitude can be corrected by the correction angle. Consequently, the correction angle can theoretically be used even for controlling the positioning device.
The correction angle itself may be determined in a wide variety of ways, some of them known, which are not the subject of this application. For example, the correction angle xcfx862 in out-of-round machining may be a function f(x, y) of the measured out-of-roundness of the surface to be machined. In the volumetric compensation, xcfx862 may be a function of one or more other functions, for example a function of the temperature, the axial expansion etc.
The invention has the great advantage that, on account of the fact that no interventions have to be made at all on the control unit itself, even finished machines can be retrofitted or converted to the new method of control without any problem.
In a preferred embodiment of the invention, the means for mixing in a correction angle xcfx862 comprise an analog phase mixer. This can access at least one, preferably two, freely programmable correction angle tables with values of the correction angles xcfx862 to be taken into account for specific states x, or their trigonometric equivalents, so that the correction angle xcfx862 is consequently not a constant value, but rather a function f(x) dependent on one or more parameters such as for example pressure, temperature, absolute position etc.xe2x80x94of the state x, that is xcfx862=f(x). Certain output signals of the position measuring device and certain intervals of xcfx861 may also be assigned certain correction angles xcfx862. For example, it may be envisaged to divide up the values of xcfx861 ranging from 0 to 2xcfx80 or 0xc2x0 to 360xc2x0 into 1024 intervals and to assign a value xcfx862 to each interval.
In an advantageous development of the invention which is distinguished by particularly fast determination of the corrected values sin (xcfx861xc2x1xcfx862), sin (xcfx861xc2x1xcfx862xc2x1p), to be passed to the control unit, the analog phase mixer has four multiplying A/D converters, one converter in each case being connected for: generating one of the following four values: sin xcfx861*cos xcfx862, cos xcfx861*sin xcfx862, sin (xcfx861xc2x1p)*cos (xcfx862, cos (xcfx861xc2x1p)*sin xcfx862. If the phase shift p between the output signals is precisely 90xc2x0, which is usually the case, sin (xcfx861xc2x1p) corresponds precisely to cos xcfx861 and the output signals consequently have the form sin xcfx861, cos xcfx861, so the converters can be wired for forming for automatically forming the values cos xcfx861*sin xcfx862, sin xcfx861*sin xcfx862, sin xcfx861*cos xcfx862 and cos xcfx861*cos xcfx862.
It has proven to be expedient if the analog phase mixer has two adding amplifiers, which respectively form, from the output values of two A/D converters in each case, one of the values sin (xcfx861xc2x1xcfx862), sin (xcfx861xc2x1xcfx862xc2x1p). The adding amplifiers then advantageously allow the output signals that have been passed from the phase mixer to the control unit to be shaped, possibly by interposing a filter, in such a way that they correspond in their characteristic exactly to the output signals supplied by the position measuring device, to the reception of which the control unit is usually set and optimized.
Alternatively, it may also be provided that the means for mixing in the correction angle xcfx862 comprise at least one A/D converter for converting the analog output signals of the form sin xcfx861, sin (xcfx861xc2x1p) of the incremental position measuring device into digital signals and a digital computing unit for calculating the values sin (xcfx861xc2x1xcfx862), sin (xcfx861xc2x1p xc2x1xcfx862), the digital computing unit than being able in turn to access at least one, preferably two, freely programmable correction angle tables, in which the values of the correction angles xcfx862 to be taken into account for specific states x, or their trigonometric equivalents, in particular the values sing and sin (xcfx862xc2x1p), are stored.
The stated object is also achieved by a method for controlling a positioning device, in particular actuators pertaining to machine tools, output signals of the form sin xcfx861, sin (xcfx861xc2x1p) being generated by an incremental position measuring device and passed to a control unit, which converts the output signals into the control signals controlling a positioning device, and correction angles xcfx862 being mixed in with the output signals before they are passed to the control unit, in such a way that the output signals obtain the form sin (xcfx861 xc2x1xcfx862), sin (xcfx861xc2x1xcfx862xc2x1p), the method being characterized in that the correction angles to be mixed in, or their trigonometric equivalents, are read out from at least one freely programmable correction angle table with values of the correction angles xcfx862 to be taken into account for specific states x, or their trigonometric equivalents.
In this case, the method may be carried out in such a way that the mixing-in of the correction angle xcfx862 is performed during the conversion of the analog output signals of the form sin xcfx861, sin (xcfx861xc2x1p) of the incremental position measuring device into digital signals, preferably using one or more multiplying A/D converters, this procedure ensuring signal processing which is particularly fast and also suitable for high input frequencies.
Alternatively, the method may, for example, be carried out in such a way that the mixing-in of the correction angle xcfx862 is performed after the conversion of the analog output signals of the form sin xcfx861, sin (xcfx861xc2x1p) of the incremental position measuring device into digital signals, preferably using a digital computing unit.
To realize the system and method according to the invention, a phase mixing device (hereafter referred to as phase mixer for short) for mixing a correction angle xcfx862 into the sinusoidal output signals sin xcfx861, sin (xcfx861+p) of incremental position measuring devices is proposed, which comprises means for reading in the output signals sin xcfx861, sin (xcfx861xc2x1p) from the position measuring device, a memory device for storing at least one table with correction angles xcfx862 to be taken into account in the state x, or trigonometric equivalents of these angles, four multiplying A/D converters for forming terms to be taken into account according to the addition theorem for the sums of/differences between two angles, and also two adding amplifiers, each connected to two of the multiplying A/D converters, for forming the values sin (xcfx861xc2x1xcfx862) and sin (xcfx861xc2x1xcfx862xc2x1p) from the values supplied by the multiplying A/D converters.
Alternatively, the phase mixing device may also be designed in such a way that it comprises means for reading in analog sinusoidal output signals sin xcfx861, sin (xcfx861+p) of an incremental position measuring device, a memory device for storing at least one table with correction angles xcfx862 to be taken into account in the state x, or trigonometric equivalents of these angles, at least one A/D converter for converting the sinusoidal analog output signals into digital signals, and la digital computing unit for forming the values sin (xcfx861xc2x1xcfx862) and sin (xcfx86xc2x1Pxcfx861xc2x1xcfx862) or cos (xcfx861xc2x1xcfx862) from the digital signals supplied by the at least one A/D converter.